The present invention relates generally to ablation catheter systems that use electromagnetic energy in the microwave frequencies to ablate internal bodily materials. More particularly, a microwave power supply for use in conjunction with an ablation catheter is disclosed which includes a tuner for impedance matching of the power supply and catheter microwave transmission line components in order to minimize reflected power and maximize catheter to tissue coupling.
Catheter ablation has recently become an important therapy for selected patients with certain arrhythmias. Two of the most common ablation approaches are: 1) to use high voltage, direct current defibrillator discharges; and 2) to use radio frequency (RF) energy as the ablating energy source. Direct current ablation has several drawbacks including the need for general anesthesia and explosive discharges leading to dangerous barotrauma effects. The problem with RF energy is that the lesion size is limited. Accordingly, in order to ablate sufficient cardiac tissue to perform the operation, it is often necessary to make repeated lesions. Although this is not necessarily dangerous, it is inefficient and often unsuccessful.
In view of the drawbacks of the traditional catheter ablation techniques, there has recently been an interest in using microwave energy as an ablation energy source. The advantage of microwave energy is that it is much easier to control and safer than direct current applications and it is capable of generating substantially larger and deeper lesions than RF catheters, which greatly simplifies the actual ablation procedures, and increases versatility by allowing treatment of supra ventricular and other previously inaccessible arrhythmogenic tissues.
In U.S. Pat. No. 4,641,649, Walinsky et al. disclose a medical procedure for the treatment of tachycardia and cardiac disrhythmia which uses microwave frequency electrical energy to ablate selected cardiac tissue. The microwave energy is transmitted over a coaxial transmission line having an antenna at its distal end. A procedure is disclosed in Langberg et al's article entitled "Catheter Ablation of the Atrioventricular Junction Using a Helical Microwave Antenna: A Novel Means of Coupling Energy to the Endocardium," PACE, pp. 2105-2113 Vol. 14 (1991). As suggested in the title, the Langberg et al. article proposes the use of a helical microwave antenna at the distal end of the catheter in order to improve the catheter's power delivery characteristics. Both of these disclosures discuss potential uses of microwave based ablation catheters and are incorporated herein by reference.
In coronary applications such as those discussed in the Walinsky and Langberg references, the catheter diameter is typically limited to approximately 71/2 French (approximately 2.5 mm in diameter). One problem that arises when using the very small diameter transmission lines that are necessitated by such diameter limitations is that the attenuation is quite large over the length of the transmission line. More troublesome is that during use, this attenuation can result in significant heat generation in the transmission line and catheter. Also of significant challange is that the impedance of the catheter to tissue coupling will vary with the location at which the catheter tip is placed in the heart. During the course of a typical ablation procedure, tissue changes and heating of the transmission line components will also effect the impedance of the catheter as viewed by the power supply.
In a typical microwave ablation catheter system, it is important to match the impedance on the catheter side with the impedance on the microwave generator side. However, the impedance on the catheter side tends to vary a fair amount as the catheter is moved about during use and as tissue properties change during an ablation procedure. This is generally due to a combination of several factors, the most notable of which seem to include heating within the catheter and changes induced as the target tissue is ablated. When the impedance changes, an increased percentage of the power is reflected and the catheter's performance is reduced. By way of example, in a representative application wherein the transmission line is approximately one meter long and is a coaxial transmission line having a diameter of 72 thousandths of an inch (1.8 mm), the power output of a well tuned system may only be in the range of 25-30% of the input power. Of course, the power output is likely to improve as the technology develops, but attenuation is always likely to be a significant concern.